The invention relates to a circuit arrangement for supplying a lamp, comprising
input terminals for connection to the terminals of an AC voltage source, PA1 rectifying means coupled to the input terminals and having output terminals for rectifying an AC voltage supplied by the AC voltage source, PA1 a first branch interconnecting the output terminals of the rectifying means and comprising a series arrangement of a first unidirectional element and first capacitive means, PA1 a second branch shunting the first capacitive means and comprising a series arrangement of a first and a second switching element, PA1 a first control circuit for rendering the first and the second switching element conducting and non-conducting, PA1 a load circuit having terminals for accommodating a lamp and a first end of which is coupled to a common point N1 of the first and the second switching element, and a second end of which is coupled to one of the output terminals of the rectifying means, PA1 a third branch connecting a point N6 of the load circuit to a point N7 between one of the output terminals of the rectifying means and the first unidirectional element, PA1 a fourth branch shunting the first unidirectional element and comprising a third switching element, PA1 a second control circuit for rendering the third switching element conducting and non-conducting.
A circuit arrangement of this type is known from the non-prepublished European patent application no. 97202122.4. In the known circuit arrangement, the first unidirectional means and the third branch jointly form part of power feedback means. With the aid of these power feedback means, a part of the power consumed by the load circuit during operation of the lamp is fed back to the output of the rectifying means. It is thereby achieved in a relatively inexpensive manner that the power factor of the circuit arrangement at the nominal luminous flux of a lamp fed by means of the circuit arrangement is relatively high.
However, if the luminous flux of the lamp is reduced by reducing the power consumed by the lamp, the ratio between the power consumed by the load circuit and the feedback power would be modified if the third switching element and the second control circuit did not form part of the circuit arrangement.
Consequently, the voltage across the first capacitive means could increase to such an extent that this might cause, for example, damage to the first and the second switching element. However, in the known circuit arrangement, the ratio between the power consumed by the load circuit and the feedback power is adjusted by adjusting the duty cycle of the third switching element. When the third switching element is conducting, the power feedback means are switched off. By means of a control loop coupled to the first capacitive means and forming part of the second control circuit, the duty cycle of the third switching element is adjusted in such a way that the voltage across the first capacitive means is substantially independent of the luminous flux of the lamp. Damage of the circuit arrangement caused during dipping of the lamp is prevented in this way. A drawback of the known circuit arrangement is, however, that the second control circuit is relatively complex and thus also expensive.